A system for controlling the primary current of an ignition coil using a constant current signal is illustrated in FIG. 1. This system comprises a rotation sensor 1 for sensing the rate of rotation of an engine, a variables sensor 2 for sensing variables other than the rate of rotation of the engine, a calculation circuit 3 for calculating the current flowing period from the data supplied from the rotation sensor 1, the variables sensor 2 and an igniter circuit 4, an igniter circuit 4, an ignition coil 5, a distributor 6, and a spark plug 7.
The changes in the primary current of the ignition coil are illustrated in FIGS. 2 and 3. The wave forms of the signal T.sub.on for controlling on-off state of the primary current I.sub.1 of an ignition coil, the signal S.sub.c detecting the constant current value of the primary current I.sub.1 of an ignition coil and the primary current I.sub.1 itself are illustrated in FIGS. 2 and 3. In the case of FIG. 2 where a sufficient OFF time, such as 5 msec, of the primary current is maintained so as to enable a complete discharge of the stored energy in the ignition coil, the delay t.sub.2 of the moment of turn-on in the wave form S.sub.c corresponding to the delay t.sub.1 of the moment of turn-on in the wave form T.sub.on is equal to the delay t.sub.1, i.e. t.sub.1 =t.sub.2.
Contrary to the above, in the case of FIG. 3 where sufficient OFF time of the primary current is not maintained so as to leave some amount of energy stored in the ignition coil, the delay t.sub.4 of the moment of turn-on in the wave form S.sub.c corresponding to the delay t.sub.3 of the moment of turn-on in the wave form T.sub.on is greater than t.sub.3, i.e. t.sub.4 &gt;t.sub.3. This is because the primary current I.sub.1 jumps by the value J the moment the primary current starts to flow because of the release of the remaining energy of the ignition coil.
An example of prior art system for controlling the primary current of an ignition coil using a constant current signal is illustrated in FIG. 4. A present constant current time T.sub.e is calculated in the routine 31'. An actual constant current time T.sub.a in the preceding ignition cycle is supplied from the routine 32'. A predetermined OFF angle, for example, 45.degree. C.A. for a four cylinder four cycle engine, which claims priority is maintained by the routine 37'. A signal T.sub.on for controlling the present ignition cycle is produced from the routine 37'. A signal of ON time for the preceding ignition cycle T.sub.on.prec is obtained at the output 10' of the routine 35'.
The difference between t.sub.e and T.sub.a is calculated in the routine 33' to produce a difference value "T.sub.e -T.sub.a " at the output 9' of the routine 33'. A value T.sub.on.prec, which is an ON time for the preceding ignition cycle, is added to the difference value "T.sub.e -T.sub.a " by the routine 36' to produce a signal, which represents an ON time for the present ignition cycle, at the output 11', After maintaining with priority a predetermined OFF angle by the routine 37', a signal T.sub.on for controlling the present ignition cycle is obtained at the output of the routine 37'.
However, the system of FIG. 4 has a disadvantage that, when the voltage of the power source which supplies power to the primary winding of the ignition coil is reduced, an extended time is required to attain the state of constant current, and hence, the ON time of the primary current of the ignition coil is increased. In this case, if the speed of rotation of the engine is increased, sufficient energy for effecting ignition is not obtained because of the maintenance of the priority claiming OFF angle.
Another example of a prior art system for controlling the primary current of an ignition coil using a constant current signal is illustrated in FIG. 5. The system of FIG. 5 is the same as the system of FIG. 4, except that a routine 38' for maintaining with priority a predetermined OFF period replaces the routine 37' of FIG. 4 for maintaining with priority a predetermined OFF angle. The routine 38' ensures an OFF period which is the period from a sparking to the next start of the primary current of the ignition coil. The system of FIG. 5 has an advantage over the system of FIG. 4, because the OFF period does not depend upon the speed of rotation of the engine, and hence, no such disadvantage as described above is involved. The OFF period is selected to be, for example, 0.6 to 0.7 msec.
However, the primary current of the ignition coil I.sub.1 jumps by a value J at the end of the OFF period, and hence, the constant current period is extended by a value t.sub.4, as illustrated in FIG. 3. This extension of the constant current period is disadvantageous because the gain of the control loop of the system of FIG. 5 becomes greater than unity, which causes an occurrence of oscillation in the control loop.
The present invention is directed to eliminating the above described disadvantages in the prior art systems.